(a) Technical Field
The present invention relates to a direct-drive electric machine. More particularly, it relates to large-size and high-torque direct-drive permanent magnet electric machines for wind turbine generators, tidal-current power generators, hydraulic power generators, and motors.
(b) Background Art
Generally, direct-drive wind turbines consist of rotor blades, generator, and fixed part structures.
FIG. 1 illustrates a typical low-speed and large direct-drive wind turbine (wind generator). A direct-drive wind turbine 10 consists of rotating parts such as rotor blades 11 and a generator rotor 12, and stationary parts such as a shaft 13, a generator stator 14, and bearings 15 that enable the rotation of the generator rotor on the shaft 13.
The generator rotor 12 is directly connected to the rotor blades 11. Thus, the generator rotor 12 rotates at low speed as the rotor blades 11.
Accordingly, a high torque generator that has to handle a high tangential force and that has a large diameter is required for large direct-drive wind turbines. Therefore, large direct-drive generators have disadvantages such as large size, large mass and high cost in order to get high torque rating comparable to geared generators. These characteristics become more remarkable when the size and the output power of wind turbines increase, i.e., up-scaling.
It is known that the direct-drive generators are disadvantageous in cost but advantageous in energy yield and reliability compared to geared generators. However, as described above, if the mass and the cost of direct-drive generators significantly increase in accordance with the up-scaling, there is a limitation in applying an existing direct-drive generator technology to large wind turbine systems.
Accordingly, a new direct-drive machine technology is urgently needed to overcome the limitation of a typical large direct-drive machine technology.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.